Fingerprint sensors that measure the fingerprint pattern using electric field sensing methods have become established. Fingerprint sensors that are based upon electric field sensing methods measure the fingerprint pattern by establishing an electric field between the finger and the sensor array, and measuring the spatial fluctuations in field strength at the sensor array caused by the shape of the fingerprint ridge and valley pattern.
In some recent applications, the sensor may desirably capture images of fingerprint patterns from fingers that are farther away from the sensor array than is typical with today's technologies. Unfortunately, as the finger gets farther away from the sensor array (for example when a relatively thick dielectric lies between the sensor array and the finger) the spatial field strength variations that represent the fingerprint pattern become weaker. One way to compensate for this loss of spatial pattern strength is to increase the voltage of the signals that generate the field between the finger and the sensor array. The fingerprint spatial pattern strength increases proportionately.
There may be limitations, however, on how much voltage can be placed on the finger and on the sensor array as well. When the voltages on the finger are too high, certain persons with very sensitive fingers may feel that voltage as a slight tingling. This may be undesirable in a consumer product. On the other hand, when voltages are too high on the sensor array, the sensor readout electronics may not perform adequately, for example, they may saturate and generate unacceptable noise, and may even be damaged.
Amplifier and processing stages that read and process the detected signals from the user's finger may limit the detected signal. As these stages are cascaded together, DC offsets inherent in each stage may not fully utilize the dynamic range of the signal. More particularly, the DC offsets may accumulate from each stage limiting the dynamic range of the desired or detected signal. There are several techniques to reduce DC offsets. For example, individual stages may have inherent trimming or calibration cycles, and sampled data systems may include built-in DC sensing and correction cycles. Other techniques may include correlated double sampling using two samples of a signal where any DC offset present in both samples is subtracted out. Band pass filtering of the signal attenuates DC and other undesired low frequency errors.